Pulse transformers



Feb. 11, 1958 M. .1. GEROULO ET AL 2,823,362

PULSE TRANSFORMERS Filed Feb. 19, 1954 INVENTOR5 ATTORNEYS Unie PULSE TRANSFORMERS Application February 19, 1954, Serial No. 411,537

Claims. (Cl. 336-96) This invention relates to pulse transformers and more particularly to hermetically sealed impregnated pulse transformers of exceptional life and reliability of electrical characteristics.

In electronic equipments utilizing pulses of electrical energy, e. g. radar and computers, pulse transformers have suffered from lack of continuing reliability of electrical characteristics and mediocre operational life. Further required of the pulse transformer is extremely fast repetition rates, ability to transmit extremely short pulses with substantially no distortion and relatively high pulse permeabilities. A portion of these characteristics have been met by the utilization of a non-metallic ferromagnetic material as the core member which cores have been of the configurations commonly known as the E and Pot" types. These latter devices still primarily suffer from unreliability of electrical characteristics and mediocrity of lifetime.

It is an object of this invention to overcome the foregoing and related disadvantages. It is a further object to produce a hermetically sealed pulse transformer of much increased operational life, thus avoiding the service difiiculties associated with the complex electronic gear. A further object of this invention is to produce a hermetically sealed pulse transformer having operational reliability and characterized by a high pulse permeability.

The objects herein have been achieved in accordance with this invention by the preparation of a pulse transformer comprising an insulated toroid of non-metallic ferromagnetic material, a plurality of insulated conductors wound about said toroid, hermetically sealed container means enclosing said toroid, terminal means for said conductors exterior of said container means and a hydrophobic slightly magnetostrictive dielectric impregnant confined within said container means. More particularly, the objects of this invention have been achieved by the production of a pulse transformer comprising an insulated toroid of non-metallic ferromagnetic material, a plurality of insulated conductors wound about said toroid, said toroid insulation having a dielectric constant of at least 1 /2 times that of said conductor insulation, container means hermetically sealing toroid and associated windings, terminal means for said conductors exterior of said container means, and a wax having a melting point of from 90 C. to 125 C. substantially filling the volume defined by said container means. A preferred embodiment of this invention is a pulse transformer comprising a lacquered toroid of a ferrite having a pulse permeability of from 400 to 500 for .l microsecond pulse, a plurality of enamel insulated conductors wound about said toroid, supporting means of a low dielectric constant thermoset resin maintaining said toroid in a fixed position, a cellulose lined metallic can having glass secured terminals hermetically sealing said toroid, said conductors terminated by said glass secured terminals, and a wax having a melting point of from 90 C. to 125 C. substantially filling said volume defined by said can.

States Patent 0 f Patented Feb. 11, 1958 'ice This invention is featured by a distribution of the electrical flux within a pulse transformer so as to position the non-metallic ferromagnetic material in a region of low voltage stress which thus avoids initiation of conduction within the toroid which in turn works up to substantial conduction and breakdown of the device. A further feature of this invention is the impregnation of the pulse transformer with an insulating substance which is hydrophobic, slight to non-magnetostrictive and of a low dielectric constant. It has been discovered that the incorporation of a number of features, including the foregoing, into pulse transformers, as they are presently known, yields a device which is extremely reliable over a much extended operational life and whose characteristics appear to be much more than the cumulative effect of the features.

One of our initial discoveries was that for reliability of operation over extended periods it was necessary to confine substantially all the magnetic flux to the regions adjacent to the windings. Introductions of an air gap such as were inherent with the presently used E and Pot" type cores so distorted and extended the flux away from the windings that the output was unreliable in that variations in temperature, shock, vibration and other external forces were transmitted to the inductive assembly. The toroid avoided the above difficulties as substantially all the magnetic flux was confined within the core. For electrical characteristics the core material was chosen to be the non-metallic ferromagnetics commonly known as the ferrites. The use of the ferrite in toroid configuration facilitated a further discovery that the non-metallic ferromagnetics suffer from the defect that when substantial potentials are imposed upon it conduction is initiated and continued stress of that magnitude avalanches said conduction until the unit undergoes breakdown and further that the susceptibility to breakdown of the device is intimately woven into the penetration of moisture into the component. Our structure has thus been produced so that the stress of imposed potential is primarily concentrated on dielectric materials other than the non-metallic ferromagnetic material which has much extended the operational life over present transformers. Any advantage obtained through such a system can well be masked by the presence of moisture within the device which has been avoided by the expedient of hermetically sealing the structure in a fully impregnated container, the impregnant of which is hydrophobic. Of substantial influence upon the reliability of the electrical characteristics of the device is the above mentioned impregnant, for it was further discovered that the aforesaid impregnant must have a negligible magnetostrictive coefficient so as to avoid imposing undue stresses and strain upon the ferromagnetic core member. Surprising as it was, we found that indeed there are suitable impregnants which are hydrophobic, of high resistance, low dielectric constant, temperature stabile and slight to non-magnetostrictive, of which wax is preferred.

Fig. 1 shows an axial cross-section of the preferred form of the device of our invention as taken on line 11 of Fig. 2. Fig. 2 shows a radial section of the device of our invention including the impregnant in a modified form.

Reference should now be made to Fig. 1 which portrays a cross-section of the preferred device of our in vention. The basic element of the pulse transformer is the non-metallic ferromagnetic toroid -2 about which are wound a plurality of windings, in this case the input conductor 4 and the output conductor 6. These leads are connected to their respective terminal posts 8 and 10. The posts are connected to an insulator supporting member 12 which has a raised central portion 14 onto which rectangular shape and of thickness of about one thirtysecond of an inch. The center was drilled so that central portion 14 could be inserted so as to provide means for positioning the toroid 2. Thesupportingmer'nber 12 and the central portion 14 are preferably of a molded thermoset resin of relatively low dielectric constant. The input and output terminal posts 8 and '10, respectively, are in turn electrically connected 'to'the exterior terminals 16 and 18. The entire assembly is positioned in a-metallic container 2% which is hermetically sealed after impregnation by bringing the respectiveleads of the assembly outwardly by means of the terminals 16 and ISthrough' the Kovar glass insulated cover 19. The container 20 is thus a cylinder with each end sealed by the "glass insulator cover in a manner well-known inthe capacitor art. The interior surface of the can has 'disposedincontactwith it a layer of kraft paper 22 which provides a positioning means for the supporting base'12 so as to'avo'id physical contact of the enclosed assembly and the "can'Q-fl. Our entire assembly is thus seento'be substantially completely impregnated with a hydrophobic material; not shown in Fig. 1, and contained in a hermeticallysealed structure in such a fashion that the potential gradient'isminimized in the region adjacent to the core member.

As indicated in the foregoing,'the'ferrornagnetic material having satisfactory electrical'prope'rties for'use as a core material for a pulse transformer, is the non 'metallic type, commonly known as ferrites. The"preferred"ferrites include the manganese zinc andnickeltypehaving a Curie point in excess of 130 C. and anin'i-tial' permeability of from 500 to 1200. "Other important electrical characteristics include a pulse permeability of atleast 480 f r a .l microsecond pulse and a frequency response up to 10 megacycles. Commonly available ferrites'suitable for such use are Ferroxcube 3C'soldby 'Ferroxcube Corporation of America, FerramicH sold byG'en'eral Ceramics, and Ceramag N sold by 'Stack'p'ole, "all of which are manganese zinc ferrite. T he electrical characteristics set forth rather than the chemical material of the core material establishes "a criteria for judging *the applicability' of the ferrom'agnetic'material to this invention. It is, however, one'of the features of our invention that a suitable core'inaterial be utilized in a certain configuration, 'namelyas a toroid or form presenting a substantially closed path for induced magnetic flux.

It was not realized by us until our discovery that'the insulation of the ferromagnetic core could indeed" exert a substantial influence :upon'the operational life available with the used suchmaterial. Realizing finally'that' the imposition of a substantial potential across the "ferromagnetic material initiated conduction 'and "accelerated the breakdown of"the"co're,it thus became necessaryto protect the core from excessive amountsofimposed potential whichwe have accomplished byinsula'tin'g to a substantial thickness'the exterior'of the core with a' dielectric of high breakdown voltage and resistance and further by imposing a plurality of'dielectrics between the tWo points of potential of'the assemblyso as toimpose upon the ferromagnetic core a "small segmentof the overall potential developed'in the component. This was accomplished by the'expedient oflacquering' the core with a material of highendielectric constant'than the impregnant and the insulation ontheprimary' conducfors and which lacquer in our"preferred'embodirnent :has a dielectric constant in excess of 4. The preferred-lacquer thus includes the copolymer of vinyl chloride *and vinyl acetate and must-he oin-thickness within-the range-of from .2 mil to (TS-mil. This higlrdielectrieconstantinsulation coated-':on- -the err omagnetic materiah 'of dielectric constant ltl sol:d1v1dedi the field that only asmall portion of the-total 'potential is placed across "or adjacent to the ferromagnetic 'material. Asindicated above,.the primary andsecondary conductors a-reinsulated-withan outer low dielectric constant-layer. This dielectric'layer serves both the function of avoiding the shorting of the adjacent turns on the core and absorbing in combination with the impregnant the major amount of the electrical stress imposed upon the dielectric materials positioned within the field of electrical stress. The conductors are coated with an enamel having a dielectric constant of from 2 /2 to 3 as contrasted with the lacquer imposed upon the surface of the ferromagnetic toroid having a dielectric constant'in excess of 4. As representative lacquers any of the various cellulose esters might be used, for example, cellulose acetate, cellulose nitrate, cellulose butyrate; etc.

As indicated in the foregoing portion of the specification, the impregnant'cooperates with the'other features to yield a pulse transformer of much improved characteristics over those presently known. The impregnant serves both as a barrier to moisture penetration which penetration has been found to be extremely detrimental to the breakdown resistance'ofthe component, has a low dielectric cohstant, a high-resistance and excellent ability to withstand electrical stress, and has slight to no magnetostrictive characteristics. Magnetostriction by the impregnant has been found to degrade the output of the pulse transformer and cause unreliability as the magneto striction of the impr'egnant so stresses'the core that its electrical characteristics arealtered. Wax meets these requirementsand if of a melting pointfof from C. to C.-",-is'-'our preferred impregnant. The low temperature li-mi. is imposedby the operating temperature for it mustbe-in-the solid phaseand upper limit imposed so as toes did shrinkage after impregnation which can result in substantial undesired voids. Although Wax forms *an integral part of our preferred structure, as shown in Fig. 2 it might well and is for certain applications filled with a-ferromagnetic particular material 23 which serves to confine further any stray flux to a region close to "the toroid Zpr'oper.

As many-apparently widely different embodiments of this invention'may be made without departing from the spirit and-scopehe'reof, it is to 'be understood that the invention is not limitedto the specific embodiments hereof except as defined in the appended claims.

What is claimed is:

1. A pulse transformer comprising an insulated toroid of non-n1etailic ferromagnetic material, aplurality of insulated conductors wound about said toroid, hermetically sealed container means enclosing said toroid, said conductors having' terminals sealed through the wall pertions of the container means, and a high'resistance hydrophobic dielectricimpregnantno more than slightly magnetostrictive confined within saidcontainer means, said impregnan't having a dielectric constant lower-than said toroid insulation and a melting point'atleast'as high as 90 C.

2. A pulse transformer comprising an insulated toroid of non-metallic ferromagnetic material, a plurality of insulated conductors wound about said toroid, said toroid insulation having a dielectric constant of at least 1 /2 times that ofsaidconductorinsulatiom'terminals for said conductors, container means hermetically sealing said toroid and associated windings but leaving the ends of the terminalsprojectingoutside said container means, and a high resistance-wax having a melting point of from 90 C. to 125 C. and a-dielectric constant lower than said toroid'ins'ulation substantially filling the volume deiined by said container means.

3. A pulse'tran'sformer comprising a lacquered toroid of a ferrite having a pulse permeabiiity'of from 400 to 500 for .l microsecondpulse, a plurality ofenainel insulated conductors wound about said toroid, supporting means of a low dielectric constant therrnoset resin maintaining said toroid in a fixed position, a cellulose linen metallic can having glass secured-terminals hermetically sealing said toroid, said conductors terminated by said glass secured terminals, and a Wax having a melting point of from 90 C. to 125 C. substantially filling said volume defined by said can.

4. A pulse transformer comprising a toroid of nonmetallic ferromagnetic material, a dielectric coating having a dielectric constant of at least 4 around the toroid, conductor windings interlinked with the toroid, a low dielectric constant, high resistance hydrophobic, dielectric solid no more than slightly magnetostrictive impregnated in and encased around the assembly, said solid having a melting point of at least 90 C. and a lower dielectric constant than said toroid coating, and a metallic container hermetically sealed around the encased assembly.

5. The combination of claim 4 in which the dielectric solid is filled with ferromagnetic particles.

References Cited in the file of this patent UNITED STATES PATENTS 844,578 Cavanagh Feb. 19, 1907 1,763,115 Wermine June 10, 1930 2,313,379 Wood Mar. 9, 1943 2,467,101 Walters Apr. 12, 1949 FOREIGN PATENTS 872,593 Germany Apr. 2, 1953 

